A number of video coding standards have recently been specified. A main purpose of these standards is to compress data to reduce the bandwidth required to transmit video signals over a communications link. State of the art ISDN video telephony terminals typically use the H.261 video coding algorithm specified by the Technical division of the International Telecommunications Union (ITU-T), whilst terminals for use with the proposed third generation 3GPP mobile networks will use the H.263 video coding algorithm specified by that same body. It will be necessary to ensure interworking of these two terminal types. The current proposals achieve interworking by providing in the communication networks a transcoder which uses a “brute force” method to first of all completely decode a video signal coded using a first of the algorithms (H.263 or H.261), and then to recode the decoded signal using the second of the algorithms. Transcoding between other video coding standards also often requires a similar brute force method. Transcoding in this way requires a great deal of processing power in the transcoder (where a great many signals may require simultaneous transcoding) which is costly in terms of network hardware. Transcoding is also time consuming, increasing transmission delay and degrading the perceived quality for delay sensitive applications such as conversational video telephony.